What, then, is the explanation of this so sudden development? Probably it lies in a migration of scholars to Oxford at this time. The migratory habits of mediæval masters and scholars are familiar to everyone who has the smallest acquaintance with the history of the Universities. The Universities of Leipzig, Reggio, Vicenza, Vercelli, and Padua, for instance, were founded by migrations from one University or another. The story of Oxford itself will furnish instances in plenty of the readiness of the University to threaten to migrate and, when hard pressed, to fulfil their threat. Migrations to Cambridge, Stamford, and Northampton are among the undoubted facts of our history. Such a migration then would be in the natural course of things, though it would not satisfy the pride of the inventors of the Alfred myth. But a migration of this kind did not take place without a cause. A cause however is not to seek. At this very period the quarrel of Henry II. with Thomas a Becket was the occasion for a migration from Paris, the ordinary seat of higher education for English ecclesiastics. A letter from John of Salisbury to Peter the Writer in 1167 contains this remark: "France, the most polite and civilised of all nations, has expelled the foreign students from her borders." This, as Dr Rashdall suggests, may possibly have been a measure of hostility aimed by the French King against the oppressor of Holy Church and against the English ecclesiastics, who as a body sided with their King against their not yet canonised primate. Henry II., on the other hand, took the same measures to punish the partisans of Becket. All clerks were forbidden to go to or from the Continent without leave of the King, and all clerks who possessed revenues in England were summoned to return to England within three months, "as they love their revenues." This would produce an exodus from Paris. A large number of English masters and scholars must have been compelled to return home. According to the usual procedure of mediæval students they were likely to collect in some one town and set up under their old masters something of their old organisation. These ordinances were promulgated between the years 1165 and 1169. The ports were strictly watched in order to enforce this edict.